You problem sounds like a "Blend Door" issue, you have these doors that open & close and everything in between to adjust air temp. These doors have a "peg" that is connected to an arm or wire just like on a ten speed bike to shift gears, that peg may have broken off.

Another issue may be a stuck open thermostat, if you have a temp gauge is it running at normal temp? If the thermostat is stuck open then heat may become an issue.

I have found several 2007 Chevrolet Impala blend door issues here in fixya.com.

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You first want to check that you have the proper level of coolant. The air blowing over the engine as you drive faster speeds is enough to help cool the engine, however at stop the heat just lingers around the engine. If the coolant is slightly low I would expect to see this. Coolant is also what is used to heat the car, so if it was low it also make sense that you may get gaps in heating the cabin. Hopefully the coolant is just low and this is an easy fix.

You're going to have to fix whatever is leaking, it will only get worse-your cooling system trys to build pressure because pressurized systems have higher boiling points. Since it cannot build pressure, it has a lower boiling point, and that will eventually cause big trouble. It also has the added effect of making the leak larger, creating the possiblilty of catastrophic failure-overheating and destroying your engine. The fact that you can see where it is leaking from should make it easy to get it diagnosed and repaired. It also is probably the reason you have no heat-there is air in the system, and it is preventing proper circulation of coolant into the heater core, hence no heat.

Have you checked the coolant level? What you describe is exactly what happens if the coolant level is low. When stopped at idle, hot coolant is not circulating through the heater core, so no heat. As the engine increases in rpm's when you pull away from the light, the water pump is spinning quicker, and some coolant is making it to the heater core. So check the level, and if it is low, keep an eye on it to see if it drops again. That of course indicates a leak somewhere. Get it fixed, as overheating the engine leads to costly repairs and worse case a new engine. Let me know how you make out with this.

To stop my heater from getting cold whenever I came to a stop, I fixed the problem by adding water to the coolant system with the car running. I added water and waited for about 30 seconds to watch the water go back down and I continue this process until the coolant reservoir was full and stop going back down. I now have no problems with my heater and it even gets hotter than before I did this process.

I had the same problem with the heater only working when accellerating. Found the overflow tank pick up tube was kinked. This of course made my coolant level low. So when the cars hot the coolant would push into the overflow, but when the car cooled the kink would prevent it from flowing back to the radiator.